CIE L*a*b* Color Measurement for TiO2
Every TiO2 CoA reports CIE L*, a*, b* values. Here's how to read them and compare grades meaningfully.
The CIE L*a*b* color space is the international standard for color measurement, defined by the International Commission on Illumination (CIE) in 1976. For TiO2 quality control, three values are reported on every CoA: L*, a*, and b*. Several derived metrics (Whiteness Index, Yellowness Index) are sometimes also reported.
L* (lightness): - Range: 0 (black) to 100 (perfect white) - TiO2 typical: 97.5–98.8 - Higher L* = whiter pigment - Differences below 0.5 L* are typically not visible to the eye - L* below 97 indicates significant impurity or off-spec material
a* (red-green axis): - Negative values = green tint - Positive values = red tint - TiO2 typical: -0.5 to +0.5 (essentially neutral) - TiO2 rarely shows significant a* deviation; this is the least useful of the three for TiO2 QC
b* (yellow-blue axis): - Negative values = blue tint - Positive values = yellow tint - TiO2 typical: +1.0 to +2.5 (slight yellow undertone) - The most important color parameter for TiO2 QC — affects perceived whiteness - Chloride-process TiO2: typically b* 1.5–2.0 - Sulfate-process TiO2: typically b* 2.0–2.5
Why b* matters more than L*: The human visual system is more sensitive to chromaticity (color shift) than to brightness. A 0.3-unit reduction in b* (less yellow) is more visible to a consumer than a 0.3-unit increase in L* (slightly brighter). Premium paint manufacturers specify tight b* limits for this reason.
Whiteness Index (WI): - Calculated from L* and b*: WI = L* - 3*b* (simplified) - Range typically 75–90 for TiO2 (higher = whiter perception) - Sometimes reported using ASTM E313 or Berger formulas - Useful single-number summary
Yellowness Index (YI): - ASTM E313: YI = 100 * (Cx*X - Cz*Z) / Y where X, Y, Z are CIE tristimulus - Range typically 1–3 for TiO2 - Inverse of WI in practical terms — lower YI = whiter perception
Measurement standards: - Reference illuminant: D65 (standard daylight) - Observer angle: 10° (CIE 1964) - Measurement geometry: d/8° (diffuse/8° viewing) — the industry standard for pigment color - White standard: pressed barium sulfate plate
Practical use in TiO2 procurement: 1. For premium coatings, specify b* < 1.6 (chloride rutile required) 2. For ink and paper, specify b* < 1.4 (anatase often preferred for lower b*) 3. For batch-to-batch consistency, specify Δb* ≤ 0.3 (achievable with tier-1 chloride producers; harder with sulfate) 4. For automotive OEM, specify Δb* ≤ 0.3 AND ΔL* ≤ 0.3 AND Δa* ≤ 0.3
Conversion notes: - Sometimes older specs reference "Munsell" values — these are not directly comparable to CIE L*a*b* - "Reflectance" measurements (single-wavelength at 460 nm) are sometimes reported instead — also a different scale - Modern CoAs all use CIE L*a*b* as the standard